This invention generally relates to a system and method for detecting flaws in the interior walls of a cavity, and is specifically concerned with inspecting the interiors of the hollow metallic plugs that are presently used to seal defective heat exchanger tubes in nuclear steam generators.
Systems for inspecting the interiors of the heat exchanger tubes used in nuclear steam generators are known in the prior art. Such systems typically employ one or more eddy current probes to detect the presence or absence of discontinuities in the tube walls created by cracks or pits in these walls. In such systems, an eddy current probe in the form of a coil whose axis of rotation is oriented along the longitudinal axis of the tube is moved throughout the tube while an alternating current is conducted through the coil. The alternating current causes the coil to emanate a time-varying magnetic field which in turn induces eddy currents in the inner walls of the tube as the coil is moved axially. Because the eddy currents create a magnetic field which is opposite in polarity to the time-varying magnetic field emanated by the probe coil, the eddy currents generated in the tube apply a measurable impedance to the alternating current that fluctuates through the coil. This impedance is highest when the metal conducting the eddy current is free from discontinuities such as cracks or pits or other imperfections in the metal. However, when such imperfections are present, the eddy currents induced in the metallic walls of the tube are subjected to a resistance, which in turn causes the electromagnetic field generated by the eddy currents to impede the time-varying magnetic field generated by the coil to a lesser extent. This reduction in impedance is usually measured by means of an AC bridge circuit connected between the input and the output of the probe coil.
While such axially-oriented eddy current probe coils were capable of detecting the presence or absence of cracks or other discontinuities in the tube walls, they were not readily capable of determining either the orientation of the crack or its precise location with respect to the longitudinal axis of the tube. To address these shortcomings, surface-riding "pancake" type eddy current coil probe systems were developed. In such systems, the probe coil is made very small (about two mm in diameter or less), and is helically moved along the axis of the tube in wiping engagement of the inner wall of the tube. Because such probes are moved in accordance with a known screw pitch, cylindrical coordinates may easily be assigned to every location on the walls that the pancake probe assumes during its helical motion. Such probes are able to pinpoint the location of cracks and other flaws in the tube wall, and to create a fine-resolution picture of the orientation and extent of the crack or other flaw. Such a pancake-type eddy current probe is used in the invention disclosed in U.S. Pat. application Ser. No. 079,860 filed July 30, 1987, by Michael J. Metala and entitled "Apparatus and Method For Providing a Combined Ultrasonic and Eddy Current Inspection of a Metallic Body" and assigned to the Westinghouse Electric Corporation.
While such surface-riding pancake-type eddy current coils represent a significant advance in the art, the physical configuration of such probes has thus far limited their application to relatively smooth-walled cylindrical cavities such as the interiors of heat exchanger tubes. The structural configuration of all known surface-riding coils would render them difficult if not impossible to use within a cavity characterized by step-wise, varying diameters such as the interior of one of the hollow metallic plugs used to seal off a defective heat exchanger tube from the primary system of the steam generator. Such plugs comprise a tube shell having a closed end, an open end and a generally cylindrical interior which has been radially expanded by means of a cork-shaped expander element that has been forcefully drawn from the closed end to the open end of the plug cavity. While the expander elements in such plugs include a centrally disposed bore that leads to the upper section of the plug interior, no known surface-riding pancake-type eddy current coil is capable of being inserted through this relatively narrow bore and engaged against and helically moved around the interior walls of the upper section of such a plug. This is an unfortunate limitation, as the applicants have observed that the upper wall section of such plugs are susceptible to stress corrosion cracking as a result of the substantial tensile stresses applied to the walls of the plug shell in the upper region by the cork-shaped expander element when it is drawn from the closed to the open end of the plug shell.
Clearly, what is needed is a system and method for inspecting the interiors of the tube plugs used in nuclear steam generators so that these plugs may be replaced or repaired in the event of stress corrosion cracking. Ideally, such an inspection system should be capable of quickly, accurately, and reliably scanning all parts of the upper interior walls of a plug shell above the expander element in order to determine not only the presence of any and all flaws, but their extent and orientation within the plug shell. Finally, such an inspection system should be positionable with known robotic systems in order to avoid exposing maintenance personnel to potentially harmful radiation.